1. Field of the Invention
The present invention relates to a piezoelectric device having a function of converting electric energy into mechanic energy (hereinafter xe2x80x9celectric/mechanic converting functionxe2x80x9d), and particularly to a piezoelectric device with improved piezoelectric properties by setting the crystallographic axes of a specific crystal in the piezoelectric device to specific conditions.
2. Description of the Related Art
A piezoelectric device has a piezoelectric film between two electrodes. Lead zirconate titanate (PZT) or other piezoelectric ceramics are used for the piezoelectric film. These materials, which are indicated by the chemical formula ABO3, are known to build a perovskite crystal structure, and this crystal structure has an electric/mechanic converting function.
FIG. 6 illustrates the perovskite crystal structure of a unit cell in the piezoelectric ceramics having the above composition. As stated above, this structural figure is expressed by the chemical formula ABO3. As an example, if the piezoelectric ceramics is PZT, A may be Pb2+, B may be Zr4+ or Ti4+, and O may be O2xe2x88x92. If the piezoelectric ceramics is PZT-PMN, Mg2+ or Nb5+ is substituted for the atom B. The orientation of crystal grains is often specified relative to crystal planes.
Crystals are divided into several crystal systems. In any system, the plane containing the apex of the crystal system can be specified through its crystallographic axes. The crystal is oriented in the normal direction of any one plane to which the crystal can be set. This normal vector is specified by the value of crystallographic axes for specifying the plane corresponding to this normal line. This orientation is called the crystal direction. Planes or orientations within crystals can be indicated by using the Miller indices. When defining primitive translational vectors such as a, b, c in FIG. 6 according to the Miller indices, the crystal oriented in the plane shown by slanted lines has a (001) plane ( less than 001 greater than  orientation) and a (111) plane ( less than 111 greater than  orientation). The polarization axis direction in piezoelectric films made of PZT or other tetragonal system has the (001) orientation, and that in a rhombohedral system has the (111) orientation.
An example of a piezoelectric device having this crystal structure is disclosed in. Patent Laid-Open Publication No. Hei 3(1990)-69512, and Applied Physics Letters, 1991, Vol. 58, No. 11, pp. 1161-1163. A prior art of an ink jet type recording head using said piezoelectric device is for example U.S. Ser. No. 5,265,315.
However, a conventional piezoelectric device had inferior electric/mechanic converting function, particularly under low voltage conditions. Especially in use with portable printers, there was the need for a piezoelectric device having high electric/mechanic converting function under low voltage conditions.
This electric/mechanic converting function is evaluated by the piezoelectric effect which is the relation between the electric polarization and the stress. The piezoelectric effect can be quantified by the product of a piezoelectric charge constant d, a piezoelectric voltage constant g and the dielectric constant. Here, the piezoelectric charge constant d is the proportional coefficient between the displacement and the electric field, and the piezoelectric voltage constant g is the proportional coefficient between the electric field and the stress. The dielectric constant is the proportional coefficient between the electric field and the planar density of electric charge. In conventional arts, the piezoelectric charge constant d was low when the applied voltage was low.
The inventors of the present invention have found that inclining the crystallographic axes influences the piezoelectric effect under low voltage. Thereupon, the inventors have manufactured a piezoelectric ceramics by intentionally inclining its crystallographic axes, thereby improving the electric/mechanic converting function under low voltage.
It is an object of the present invention to provide a piezoelectric device having good piezoelectric properties under low voltage by adjusting the crystallographic axes of the (001) plane.
It is a further object of the present invention to provide an ink jet recording head which can discharge ink under low voltage well by including a piezoelectric device having the crystallographic axes of the (001) plane.
It is another object of the present invention to provide a method of manufacturing a piezoelectric device having adjustable crystallographic axes of the (001) plane.
It is still another object of the present invention to provide a method of manufacturing an ink jet recording head having a piezoelectric device with adjusted crystallographic axes of the (001) plane.
To this end, the piezoelectric device according to the present invention includes a lower electrode, an upper electrode, and a crystallized piezoelectric film arranged between the lower electrode and the upper electrode, wherein the crystallographic axes (orientation) of the columnar grain with the (001) orientation of the crystal forming the piezoelectric film is inclined by a certain angle relative to the normal direction of the lower electrode face.
The angle formed by the crystallographic axis of the columnar grain with the (001) orientation and the normal direction of the lower electrode face may be 5 degrees or more and 20 degrees or less. The columnar grain with the (001) orientation includes an interface layer with 5 nm or less thickness in the vicinity of said lower electrode. As an alternative, the crystallographic axis of the columnar grain with the (001) orientation is parallel to the normal direction of the lower electrode face within a certain distance from the interface between the lower electrode and the crystal grain. At this time, the predetermined distance in which the crystallographic axis is parallel to the normal direction of the lower electrode face is within the range of 15 nm or less from the interface between the crystal grain and the lower electrode.
On the other hand, in the grains forming the piezoelectric film, the crystallographic axes of the columnar grains with the (111) orientation are substantially parallel to the normal direction of the lower electrode face. The columnar grain with the (001) orientation has a crystal grain with larger average grain size than the columnar grain with the (111) orientation.
Specifically, the piezoelectric film preferably contains piezoelectric ceramics which is any one of lead zirconate titanate (Pb(Zr, Ti)O3:PZT), lanthanum lead titanate ((Pb, La)TiO3), lead lanthanum zirconate titanate ((Pb,La)(Zr,Ti)O3:PLZT), and lead magnesium niobate zirconate titanate (Pb(Zr, Ti)(Mg,Nb)O3:PZT-PMN).
The ink jet recording head according to the present invention includes a pressure chamber provided on a pressure chamber substrate, and a nozzle provided in the pressure chamber, wherein ink can be discharged from the nozzle by changing the volume of the pressure chamber, and volume change is caused in the pressure chamber by using the piezoelectric device according to the present invention as the drive means.
The method of manufacturing a piezoelectric device having an electric/mechanic converting function according to the present invention includes the step of using a sol with the piezoelectric ceramics adjusted to a mol concentration of 0.7 mol/1 to 1.5 mol/1 to form a piezoelectric film sandwiched between a lower electrode and an upper electrode.
The sol above may be generated by including polyethylene glycol in an amount of 0.15 mol or more to 0.5 mol or less relative to 1 mol of piezoelectric ceramics.
Also, during the step of forming the piezoelectric film, each of the thin layers is formed by a step of coating the sol at a predetermined thickness, and a step of drying and defatting such coated sol, and each time the thin layers are layered to a certain total number of layers, these layers are subjected to high-speed and high-temperature heat treatment to be crystallized.
The method of manufacturing an ink jet recording head according to the present invention, wherein volume change is caused in a pressure chamber provided in a pressure chamber substrate to enable ink to be discharged from a nozzle provided in the pressure chamber, includes the steps of forming an insulating film on the pressure chamber substrate, forming a piezoelectric device on said insulating film by the method according to the present invention, shaping the piezoelectric device to enable a change in volume to be caused inside the pressure chamber, and forming the pressure chamber on the pressure chamber substrate.